Dielectric wall stabilization of intense charged particle beams



June 6, 1967 J. BRlGGs 3,324,325

' DIELECTRIC WALL STABILIZATION OF INTENSE CHARGED PARTICLE BEAMS FiledSept. 10, 1965 2 Sheets-Sheet 1 INVENTOR RICHARD J. BRIGGS BY Km 4- QATTORNEY June 6, 1967 R. J. amass 3,324,325

DIELECTRIC WALL STABILIZATION OF INTENSE CHARGED PARTICLE BEAMS FiledSept. 10, 1965 2 Sheets-Sheet 2 I PARTICLE ORBITING REQUENCY ENERGYINVENTOR. RICHARD J. BRIGGS BY M 0. W

' ATTORNEY United States Patent 3,324,325 DIELECTRIC WALL STABILIZATIQN0F INTENSE CHARGED PARTICLE BEAMS Richard .I. Briggs, Dan /idle, Califassignor to the United States of America as represented by the UnitedStates Atomic Energy Commission Fiied Sept. 10, 1965, Ser. No. 486,57715 Claims. {(11, 3l362) ABSTRACT OF THE DISCLOSURE Apparatus for guidingan intense charged particle beam along a path defined by a guide orhousing in which an effective dielectric structure is disposed betweenthe guide and beam to stabilize the beam against negative massinstability disturbances.

The present invention relates generally to the stabilization of thetrajectory of a beam of charged particles. More particularly, itappertains to inductive wall stabilization of a charged particle beamtraveling along an arcuate path.

Beams of charged particles are extensively employed for a variety ofpurposes in nuclear research, creation of plasmas, and various relatedfields. Often it is desirable to provide an intense, i.e., of the orderof one ampere, beam of charged particles to investigate, for example,the structure of the nucleus of atoms. Moreover, in many nuclearexperiments it is generally preferred that a well focused beam ofparticles be provided characterized by a minimum spread in particleenergy to increase precision, to eliminate spurious effects as well asto simplify interpretation of experimental results.

Generally, particle beams are produced by linear or circular pathaccelerators. While circular path accelerators are able to generate afocused low intensity, i.e., milliamperes or less, beam of energeticcharged particles, with a minimum particle energy spread, the beamintensity is limited to low levels by instability effects. Similarly, inraising the current levels of beam storage rings instabilities areencountered which limit the intensity of the beams which may be stored.Beam storage ring techniques are often used in linear accelerators asWell as circular accelerators.

A predominant beam intensity limiting instability is the negative massinstability of beams directed along an arcuate path. This type ofinstability gives rise to an unwanted density modulation perturbation ofthe particle beam along the axis of travel and in some cases, results ina loss of particles to the walls of the beam guide. As will be set forthin more detail hereinbelow, when the particles of the beam areinfluenced by the negative mass instability, they collect and formhighly dense bunches spaced at intervals along the beam axis. As thebunch density increases, for example, as a result of increasing theintensity of the beam, space charge effects eventually prevent thefurther collection of particles in the bunches. Particle beams up to oneampere in intensity have been generated by circular type accelerators byintroducing betatron oscillations and/ or particle energy spread in thebeam to offset the negative mass instability influences at the expenseof beam definition and exactness in particle energy.

The present invention provides apparatus which accomplishes thestabilization of beams of particles having a narrow span of energies athigh intensities and thereby overcomes many of the limitations anddisadvantages characteristic of prior art energetic particle beamarcuate type generators and beam path housing structures or beam3,324,325 Patented June 6, 1967 guides. More specifically, stableconfinement of charged particle beams magnetically directed alongarcuate paths of generators and beam guides is achieved by providing theparticle beam guide structure with a magnetically permeable wall memberinterposed between the beam and beam facing surface of the beam guidestructure at locations on diametrically opposite sides of the arcuatesection of the beam. The material and dimension of the wall member isselected for a given size beam guide to define at predetermined beamenergies an inductive impedance along the arcuate beam path by modifyingthe electric field component therealong to allow the inductive magneticfield component to predominate. By constructing the particle beam guidestructure to present such an inductive impedance to the charged particlebeam as it traverses an arcute path, forces which otherwise are presentto drive the negative mass instability are eliminated. In the absence ofinstability inducing forces, it is unnecessary to rely on particleenergy spread and/or betatron oscillation effects to stabilize the beamwith concomitant increased particle energy spread and loss of beamdefinition.

Accordingly, an object of the invention is the provision of apparatuscapable of stably confining an intense beam of charged particlesdirected along an arcuate path.

Another object of the invention is to provide a stably confined wellfocused beam of charged particles having a minimum spread of particleenergizes. A further object of the invention is to eliminate the factorsgiving rise to negative mass instabilities in charged particle beamsdirected along arcuate paths.

These and other objects and advantages of the present invention willbecome apparent from the following description taken in connection withthe accompanying drawings in which:

FIGURE 1 is a cross sectional view of a beam guide structure constructedin accordance with the present invention illustrating the orientation ofthe electric and magnetic field vectors;

FIGURE 2 is a normalized graphical plot of particle orbiting frequencyas a function of beam energy for different classes of charged particleaccelerators, as follows:

Curve a representing weak focusing particle accelerators; and curve brepresenting strong focusing particle accelerators;

FIGURE 3 is a sectional view of a segment of a circular beam guide tubeincluding an arcuate section constructed in accordance with the presentinvention;

FIGURE 4 is an isometric view of a toroidal charged particleaccelerator;

FIGURE 5 is a cross sectional view taken along lines 55 of FIGURE 4portraying a dielectric type inductive inner Wall;

FIGURE 6 is a cross sectional view of a toroidal charged particleaccelerator with spaced apart inductive wall members;

FIGURE 7 is a sectional view of a beam guide with an inductive innerwall defined by spaced conductive annular fins;

FIGURE 8 is an isometric view of a section of one embodiment of atwo-beam type charged particle accelerator constructed in accordancewith the present invention;

And FIGURE 9 is an isometric view of a section of the beam guide of analternate embodiment of the two-beam charged particle accelerator ofFIGURE 8.

In charged particle beam accelerators and beam guides where a beam ofparticles is magnetically directed along an arcuate path, the beam ofparticles can be subject to the negative mass instability when theeffective orbital frequency of the particles is a decreasing function ofenergy. Orbital frequency is herein defined as the number of times persecond that a charged particle beam would revolve about a closedcircular path having a radius of curvature equal to that of the arcuatepath through which they are magnetically directed. Hence, chargedparticles directed through a particle beam guide having both straightand arcuate sections 11 and 12 respectively as illustrated in FIGURE 3have an effective orbital frequency in the same sense as chargedparticles directed around a toroidal particle beam accelerator 13 (orstorage ring) illustrated in FIGURE 4.

The negative mass instability driving forces arises from the inherentirregularities in the particle density distribution along the arcuatebeam axis, and the nature of the beam guide wall facing the particlebeam. Conventional beam guides of circular charged particle acceleratorsand guides are constructed of conductive material with a smooth surfacefacing the beam so that the irregular particle density distribution,i.e., longitudinal density modulation of the beam, establishes anelectric field which has a longitudinal vector component that serves todrive the negative mass instability. The physical explanation of thisinstability can best be described with reference to FIG- URE 1. Asillustrated therein, the charged particles of the beam 14 gather inbunches 16 at spaced intervals along an arcuate path 17 traversed bybeam 14. Hence, beam 14 is defined by particle bunches 16 spaced apartby void spaces 18, with the bunches 16 and voids 18 represented bypositive and negative signs respectively. As illustrated in FIGURE 1,beam 14 is composed of positively charged particles such as protons orpositive ions. However, it is to be noted that the beam 14 could becomposed of negatively charged particles such as electrons or negativeions. In such cases, the negative signs would represent the particlebunch and the positive signs the particle voids. In either case, i.e.,positively or negatively charged particle beams, the negative massinstability grows in the same manner.

Considering now the particle bunch 16 in conventional arcuate beamguides, the positive particles at the head 19 of bunch 16 experience aforward longitudinal force i.e., in the direction of arrow 17. Moreover,the positive particles at the tail 21 of bunch 16 experience a backwardlongitudinal force. Where the particles orbiting frequency is adecreasing function of energy, for example, above the transition energyin strong focusing particle accelerators, the longitudinal forcesincrease the orbiting radius and decrease the angular velocity of theparticles at the head 19 of beam 16 so that such particles move backwardtowards the region of increased density of bunch 16. Simultaneously, thelongitudinal force experienced by the particles at the tail 21 of bunch16 decreases the orbiting radius and increases the angular velocity ofthe particles and the particles move forward towards the region ofincreased density of bunch 16. As the bunch 16 becomes denser, thetendency of the particles to bunch becomes greater. Consequently, inmagnetically directed charged particle accelerators and beam guideshaving an arcuate beam path segment, beam intensity limitations areencountered at particle beam energies where the particle orbitingfrequency decreases with an increase in energy.

Circular type charged particle accelerators are generally classifiedinto two groups, weak and strong focusing types. With reference toFIGURE 2, it is seen that the weak focusing accelerators generateenergetic charged particle beams under conditions favoring the existenceof the negative mass instability. (See curve a.) This principally arisesout of the fact that such machines are characterized by a radialmagnetic field gradient in which, as the beam energy is increased, thebeam moves towards weaker magnetic field regions at larger radialorbits. Hence, the orbital frequency becomes lower.

In the case of strong focusing circular beam accelerators and guidetubes, e.g., for proton and electron beams, relativistic effects areencountered at high beam energies,

A. i.e., of the order of one mev. for electrons and one bev. forprotons, which result in the orbital frequency being lower, i.e., thecycles/ second are fewer, as the beam energy is increased. (See curveb.)

As noted hereinbefore attempts to offset the effects of the negativemass instability encountered by both classes of accelerators and guideshas involved particle energy spread and betatron oscillation techniques.As further noted hereinbefore, with the present invention an entirelynew concept for offsetting the effects of the negative mass instabilityis contemplated. In fact, with the apparatus of the present invention,the negative mass instability is eliminated.

It has been found that if the segments of the beam guide structure ofcircular charged particle accelerators and beam guides proximate thearcuate beam path represent an inductive longitudinal impedance to thecharged particle beam magnetically directed therethrough, the conditionsgiving rise to the negative mass instability will be eliminated; hencethe negative mass instability itself is eliminated. From Poyntingstheorem, it is found that such an inductive impedance at the beamresults whenever the magnetic field energy storage outside the beam isgreater than the electric field energy. By shorting out or substantiallyreducing the electric field component of the electromagnetic field inthe region of the inner wall of the beam guide, the energy stored in themagnetic field component will be greater than that stored in theelectric field component of the electromagnetic field. Shorting out theelectric field, i.e., reducing the electric field potential to zero, ata point between the beam guide and charged particle beam insures theestablishment of such an inductive impedance field.

Referring now to FIGURE 1, negative mass instability stabilization isaccomplished by disposing a magnetically permeable electric fieldintensity reducing wall member 22 of a selected wall thickness, t,proximate the inner surface 23 of arcuate beam guide 24. Such a wallmember 22 is defined herein as an inductive Wall since the modificationof the electric field, as described above, results in an inductiveimpedance stabilizing effect upon the charged particle beam. Beam guide24 is an arcuate segment of, for example, a toroidal charged particleaccelerator of the type portrayed in FIGURE 4, positioned within amagnetic field represented by arrows 26 which bends the charged particlebeam 14 along path 17 curving out of the plane of the drawing. Theinductive wall 22 is interposed between beam 14 and inner surface 23 ofguide 24 at locations on diametrically opposite sides of the beam.

The configuration and composition of inductive wall 22 required may takeseveral forms. As noted hereinbefore, stabilization of the negative massinstability will be accomplished by a proper disposition ofappropriately dimensioned magnetically permeable electric fieldintensity reducing structure of selected wall thickness. The particularwall thickness selected is dependent upon the energy range of thecharged particle beams to be directed therethrough two embodiments ofthe inductive wall of the present invention are illustrated respectivelyin FIG- URES 5 and 7. Referring first to FIGURE 5, a dielectric material27, e.g., aluminum oxide or titanium dioxide having typical dielectricconstants of 5.5 and respectively, of predetermined thickness t isdisposed to be supported by the inner surface 28 of a toroidal beamguide tube 29 of toroidal charged particle accelerator 13, the toroidaltube 29 having a selected minor radius r The dielectric material 27 ispositioned as a film or layer at locations on diametrically oppositesides of a beam 31 magnetically directed through tube 29 by magneticfield 32 to define an inductive wall. For a toroidal tube 29 of a givenminor radius r, and a charged particle beam 31 of a given energy andbeam radius r the required thickness, i of the dielectric material 27 isdefined by the inequality expression 1)" i' an where v is the particlevelocity and c is the speed of light.

As shown in FIGURE 5, the dielectric material 27 is simply disposed tocover the entire inner surface of tube 29. However, the stabilizingcondition is satisfied merely if spaced segments 33 and 34 of dielectrichaving a thickness r are positioned at locations on diametricallyopposite sides of a beam 36 as portrayed in FIGURE 6, so that a suitableinductive longitudinal electric field impedance will be present at thebeam.

Attention is now directed to FIGURE 7 wherein there is portrayed aconductive fin inductive wall embodiment of the present invention. Aswill be set forth in detail hereinbelow, the conductive fin inductivewall shorts out, i.e., reduces to a negligible level the electric fieldin the vicinity thereof thereby serving to insure that the electricfield component of the electromagnetic field is substantially reduced.As shown in FIGURE 7, a plurality of conductive annular fins 37 ofselected annular thickness t are disposed spaced apart in coaxialrelation to the beam axis 38. In one embodiment contemplated, fins 37are secured to the inner surface 39 of the arcuate section of a beamguide 41. Beam guide 41 may be a toroidal tube of a circular typecharged particle accelerator, for example, as illustrated in FIGURE 4, atoroidal tube of a charged particle storage ring device, or an arcuatebeam guide section connecting two angularly disposed linear beam guides,etc. In any case, for a toroidal tube of a given minor redius r and acharged particle beam of a given energy and beam radius r the requiredannular thickness, I of the fins 37 is defined by the inequalityexpression Furthermore, it is preferable that the fin spacing s beadjusted so that the following inequality expression is satisfied m v mwhere E is the electric field in the medium, E is the electric field ina vacuum, and c is the dielectric constant of the medium. Hencereferring again to FIGURE 1, since in the case of the dielectric typeinductive wall, c is substantially greater than unity, the electricfield therein, i.e., E will be substantially reduced from the magnitudeof the electric field 42 in the region between the inductive wall 22 andbeam 14. Furthermore, in the case of the fin type inductive wall, theelectric field lines terminate at the edge thereof facing the beam.Consequently, in the region between the fins the electric field isessentially zero. This results in an effective fin dielectric constantapproaching infinity. Therefore, from the foregoing it is seen that amagnetically permeable electric field intensity reducing wall memberinterposed between the beam guide and a charged particle beammagnetically directed along an arcuate path results in eliminating thenegative mass instability.

Although the inductive wall of the present invention will findapplication in any circular charged particle beam accelerator orcircular beam guide through which a beam is magnetically directed alongan arcuate path, the following discussion will describe the use of theinductive wall in two distinct charged particle circular type beamaccelerators. 1 Referring again to FIGURES 4 and 5, toroidal chargedparticle beam accelerator 13 comprising stainless steel toroidal beamguide tube 29 is provided with a beam entrance port 45 for injectingtherein from a charged particle source (not shown) selected chargedparticles, e.g., protons, to form a positive ion type beam 31. Toevacuate tube 29 a port 43 is adapted thereto to provide suitablecommunication to a vacuum pump (not shown) whereby the tube may beevacuated to at least l() millimeters of mercury. An aluminum oxidematerial of thickness t is disposed in surface covering relation to theinterior surface 23 of tube 29 to define inductive wall 27. The tube 29is positioned in a magnetic field 32 so that the lines thereof permeatethe tube 29 in a direction perpendicular to the plane of the tube. Instrong focusing type charged particle accelerators, the magnetic field32 is modulated to accelerate the proton beam 31 to relativisticenergies to, for example, impinge on a target (not shown) disposed incommunication with the interior of tube 29.

For a proton beam accelerated from a non-relativistic to a relativisticenergy equivalent to =25 where 7 is defined by the equation theaccelerator 13 is constructed preferably in accordance with thefollowing parameters:

With reference to curve b of FIGURE 2, it is seen that as the energy ofbeam 31 is increased from non-relativistic to relativistic energies,both positive and negative mass effects are encountered. In the positivemass regime, the efiects of the electromagnetic waves on the chargedparticles is exactly contra to those previously described as encounteredin the negative mass regime. That is to say, in the presence of aninductive wall, a beam of charged particles below the transition energy,E may encounter positive mass instability forces. However, by adjustingthe thickness, r of dielectric 27 to make the inequality expression 1)an equality at the transition energy, both positive and negative massinstabilities will be eliminated, i.e., a null point region of stabilityis in effect provided. In the strong focusing charged particleaccelerator constructed in accordance with the above noted parameters,the thickness of the aluminum dioxide inductive wall 27 defined byequation is 3.2 millimeters.

In the case where the proton particle beam is to be stored in a toroidalstorage ring constructed in accordance with the above noted parametersof the toroidal accelerator 29, magnetic field 32 is maintained constantand means 44 for accelerating beam 31 is adapted to tube 29. The beamaccelerating means 44 is operated to maintain the beam energy constant.Since single energy beams are directed through storage rings, thethickness of the dielectric inductive wall need only be adjusted toeliminate the negative mass instability at that energy. Hence, from theinequality expression (1), it is found that an aluminum oxide inductivewall thickness of 0.32 millimeter or greater will suppress the negativemass instability at a relativistic beam energy equivalent to 'y=25.

Energetic charged particle beam accelerated or stored in apparatus otherthan toroidal types also can be stabilized by an inductive wall. Forexample, referring to FIGURES 8 and 9, an annular beam guide 46 ofrectangular cross section of a two-beam strong focusing charged particleaccelerator, such as the MURA machine, is positioned between the poles47 and 48 of a radial-sectored magnet 49. Adjacent radial sectors 51 ofmagnet 49 are energized to generate opposite magnetic fields having agradient which increases radially outward. The charged particles whichare to be accelerated and formed into a beam are injected through aninlet port 52 communicating to the interior of guide 46 at its radiallyinnermost wall 53. In order to evacuate the interior of guide 46, a port54 is provided at a wall thereof for communication of the interior to asuitable vacuum pump (not shown).

In using the MURA accelerator to accelerate electrons to 40 mev., theaccelerator is constructed in accordance with the following parameters:

Cm. Inner radius of guide 120 Outer radius of guide 200 Injection radiusof particles 125 Interior vertical height of wall 53 7.0

The radial sectors 51 of magnet 49 are energized to cause the electronsto execute orbits of increasing radius whereby at 35 mev. the electronsorbit at a radius of 195 centimeters. The transition energy of the beamis 1.1 mev. and i reached by the beam as it accelerates to a radius of145 centimeters. Since the negative mass instability is encounteredabove the transition energy, it is only necessary to provide aninductive wall at the top and bottom walls, 57 and 58 respectively, ofthe annular guide proximate the regions wherein the energy of the beamis above the transition energy. Hence, in the accelerator describedabove, an inductive wall 59 is positioned within region bounded by topand bottom *wall 57 and 58 and radii of 145 centimeters and 195centimeters. Either a dielectric material or conductive ring shaped finsdisposed at opposite surfaces 61 and 62 of top and bottom walls 57 and58 respectively would adequately serve as inductive wall 59.

For example, in FIGURE 8 an aluminum oxide layer 63 is coated onsurfaces 61 and 62 of walls 57 and 58 respectively between radii of 145centimeters and 195 centimeters. The thickness of the layer 63 decreasesradially outward from a maximum of at least 0.7 centimeter at region 64to a minimum of at least 3.5 1() centimeters at region 66.

With reference to FIGURE 9, it is seen that inductive Wall 59 also couldbe formed by disposing radially spaced ring shaped aluminum oxide layers67 of varying thickness along surfaces 61 and 62. The thickness oflayers 67 varies inversely as the radius of the layers. The thickness ofthe inductive wall 59 at point locations along surfaces 61 and 62corresponding to instantaneous values of beam energy and beam orbitingradius, can be determined approximately from the inequality expression1.

It is further noted that the intensity of energetic charged particlebeams directed along arcuate paths can be further increased by combininginductive wall beam stabilization with energy spread and betatronoscillation techniques. As noted hereinbefore, the beam definition andexactness of beam energy is affected by such techniques. However, itshould be appreciated that heretofore unattainable very intense beams ofcharged particles can be generated by combining such beam stabilizingtechniques with the inductive wall beam stabilization of the presentinvention.

While the present invention has been hereinbefore described with respectto specific embodiments, it will be apparent that numerous modificationsand variations are possible within the spirit and scope of the inventionand thus it is not intended to limit the invention except by the termsof the following claims.

What is claimed is:

1. In charged particle beam directing apparatus including means forgenerating a magnetic field to direct a charged particle beam of apredetermined energy along an arcuate path, the combination comprising,a housing having at least a section thereof constructed of magneticallypermeable material to allow said magnetic field to penetratetherethrough adapted to receive said charged particle beam therethroughin a direction angularly related to said penetrating magnetic field,said penetrating magnetic field directing said beam along an arcuatepath so that said beam induces electric field regions between said pathand housing effective to reduce negative mass instabilities in saidbeam, and a magnetically permeable wall structure mounted between saidbeam and said magnetically permeable section of said housing atlocations on diametrically opposite sides of said beam, said structurehaving a thickness correlated with the energy of said beam to provide adielectric effect for offsetting said regions of induced electric field,thereby stabilizing said beam against negative mass instability.

2. The charged particle beam directing apparatus as recited in claim 1further defined by said housing being a magnetically permeable toroidaltube having an inner surface and adapted with a charged particleentrance port, said tube adapted to be supported in said magnetic fieldin a plane perpendicular to said magnetic field.

3. The charged particle beam directing apparatus as recited in claim 2further defined by said wall structure being disposed in coveringrelation to the entire inner surface of said toroidal tube.

4. The charged particle beam directing apparatus as recited in claim 1further defined by said wall structure being a dielectric material.

5. The charged particle beam directing apparatus as recited in claim 1further defined by said wall structure being comprised of a plurality ofspaced apart conductive fins mounted to said magnetically permeablesection of said housing, said fins extending within said housing aselected distance.

6. The charged particle beam directing apparatus as recited in claim 1further defined by said housing being a magnetically permeable annulusof rectangular cross section defined by radially extending top andbottom walls terminating at inner and outer end walls having an innersurface and adapted with a charged particle entrance port, said annulusadapted to be supported in said magnetic field in a plane perpendicularto said magnetic field.

7. The charged particle beam directing apparatus as recited in claim 6further defined by said wall structure being a dielectric materialdisposed on said top and bottom walls at opposite locations, thethickness of said dielectric material decreasing in the radialdirection.

8. The charged particle beam directing apparatus as recited in claim 7further defined by said dielectric material disposed to cover the entiresurface of said top and bottom walls between the outer wall andapproximately a point one third the distance from said inner Wall.

9. An energetic charged particle storage ring device comprising, amagnetically permeable toroidal tube having an inner surface and adaptedwith a charged particle entrance port for introduction and circulationof a charged particle beam wherein said beam induces electric fieldregions within said storage tube effective to reduce negative massinstabilities in said beam, and a magnetically permeable wall structuremounted between said beam and said inner surface of said tube atlocations on diametrically opposite sides of a beam directed 9 throughsaid tube, the thickness t of said Wall defined by the inequalityexpression Where v is the velocity of the particles of said beam, is thespeed of light, r is the minor radius of the toroidal tube at the innersurface, r is the radius of the beam, and e is the dielectric constantof the wall structure thereby offsetting said regions of inducedelectric field.

10. The storage ring recited in claim 9 further defined by said wallstructure being a layer of dielectric secured in covering relation tothe inner surface of said tube.

11. The storage ring recited in claim 9 further defined by saiddielectric selected from the group of materials consisting of aluminumoxide and titanium dioxide.

12. The storage ring as recited in claim 9 further defined by said wallstructure being comprised of a plurality of spaced apart annularconductive rings secured Within said toroidal tube in coaxial relationwith its minor axis.

13. A beam guide structure for a circular charged particle beam strongfocusing accelerator for circulating a charged particle beam whereinsaid beam induces electric field regions within said acceleratoreffective to reduce negative mass instabilities comprising, amagnetically permeable toroidal tube having an inner surface andprovided with a charged particle entrance port, and a layer ofdielectric material of selected thickness secured in covering relationto the inner surface of said tube, the thickness r of said dielectricdefined by the equation where v, is the velocity of the particles ofsaid beam at the transitional energy of said beam, c is the speed oflight, r is the minor radius of the toroidal tube at the inner surface,1' is the radius of the beam, and e is the dielectric constant of thedielectric thereby offsetting said regions of induced electric field tostabilize said beam against negative mass instabilities.

14. The beam guide structure as recited in claim 13 further defined bysaid dielectric selected from the group of materials consisting ofaluminum oxide and titanium dioxide.

15. Apparatus for stabilizing a charged particle beam against negativemass instability while said beam is traveling in a beam guide whereincharged particle bunches of increasing size create induced electricfield regions of increasing magnitude comprising a magneticallypermeable wall structure interposed between said beam and said beamguide at locations on diametrically opposite sides of said beam, saidwall structure having a thickness correlated with the energy of saidbeam to provide a dielectric constant effect when interacting with saidbeam which is of a magnitude effective to attenuate said electric fieldregions of increasing magnitude.

References Cited UNITED STATES PATENTS 2,825,833 3/1958 Yanagisawa 3l362JAMES W. LAWRENCE, Primary Examiner. STANLEY D. SCHLOSSER, Examiner.

1. IN CHARGED PARTICLE BEAM DIRECTING APPARATUS INCLUDING MEANS FORGENERATING A MAGNETIC FIELD TO DIRECT A CHARGED PARTICLE BEAM OF APREDETERMINED ENERGY ALONG AN ARCUATE PATH, THE COMBINATION COMPRISING,A HOUSING HAVING AT LEAST A SECTION THEREOF CONSTRUCTED OF MAGNETICALLYPERMEABLE MATERIAL TO ALLOW SAID MAGNETIC FIELD TO PENETRATETHERETHROUGH ADAPTED TO RECEIVE SAID CHARGED PARTICLE BEAM THERETHROUGHIN A DIRECTION ANGULARLY RELATED TO SAID PENETRATING MAGNETIC FIELD,SAID PENETRATING MAGNETIC FIELD DIRECTING SAID BEAM ALONG AN ARCUATEPATH SO THAT SAID BEAM INDUCES ELECTRIC FIELD REGIONS BETWEEN SAID PATHAND HOUSING EFFECTIVE TO REDUCE NEGATIVE MASS INSTABILITIES IN SAIDBEAM, AND A MAGNETICALLY PERMEABLE WALL STRUCTURE MOUNTED BETWEEN SAIDBEAM AND SAID MAGNETICALLY PERMEABLE SECTION OF SAID HOUSING ATLOCATIONS ON DIAMETRICALLY OPPOSITE SIDES OF SAID BEAM, SAID STRUCTUREHAVING A THICKNESS CORRELATED WITH THE ENERGY OF SAID BEAM TO PROVIDE ADIELECTRIC EFFECT FOR OFFSETTING SAID REGIONS OF INDUCED ELECTRIC FIELD,THEREBY STABILIZING SAID BEAM AGAINST NEGATIVE MASS INSTABILITY.